Rule of 12ths, how accurate is it?

[Neeves]

With the vagueness of tide charts modified by pressure and wind and the availability of tidal graphs - how dependable is the rule of 12ths?

We beach quite often and I use the rule of 12ths (came off a beach at noon (hence the question), its easy - but I am cautious.

Am I showing my age? Is there a better way.

Jonathan

 

[Poignard]

With the vagueness of tide charts modified by pressure and wind and the availability of tidal graphs - how dependable is the rule of 12ths?

We beach quite often and I use the rule of 12ths (came off a beach at noon (hence the question), its easy - but I am cautious.

Am I showing my age? Is there a better way.

Jonathan

Good enough for most situations where the tidal curve is conventional but not where it is abnormal as in the Solent.

 

[Refueler]

When all else fails ... its fine ...

 

[Sandy]

If its within 20 mins I am happy.

 

[Uricanejack]

You could just by a set of tide tables and use them or an Internet app. Or use the GPS tide calculator.

I would say rule of 12ths was good enough for most circumstances. Just bear in mind it’s a rough estimate and allow for error.
Even the actual tide tables adding in about 20 odd variables is just a prediction. The calculation of methods based on a sine wave,

Actual geography may change things a bit.

 

[Neeves]

I have set of tide tables, they are issued by our Bureau of Meteorogy and give me high and low tide each day against time (plus moon phases) - that's it.

Which is why I asked.

Its not the time so much, 20 mins plus or minus would not phase me - 30 cm might be more critical.

We have, roughly, 2m tides max.

Here, Pittwater, its all very conventional - NE Tasmania - different ball game (6 knot tidal flows, lows coming in of Southern Ocean etc).

Jonathan

 

[lw395]

Do you have any ports with tide gauges nearby?
In the UK we have
Real-time data – UK National Tide Gauge Network | National Tidal and Sea Level Facility
but also we hear ship pilots getting real time tide gauge readings over the VHF.
Sometimes the real tide is 30cm or more away from astro-based predictions, let alone the rule of twelfths.
And sometimes it's wrong without the weather being worthy of headlines.

If the predicted curve is a nice sine wave, the rule of twelfths is not much worse than other predictions.
And bear in mind that the predicted curve is valid for on point only, usually the HM's tide gauge, and may differ in time or height as little as half a mile away.

If you want to dry out boats it's worth studying the local tides over time. You may find local boatyard operators have a lot of knowledge, maybe just in their heads though.
They may tell you 'it's all **** different since that winter storm' unfortunately.

 

[ghostlymoron]

All the rule of twelths does is to produce an approximation of a sine wave based on high and low water heights and times. Bearing in mind that there are weather related variables anyway, it's usually good enough. But if you have a modern chart app such as navionics, you can get a minute by minute prediction from that with out the math.

 

[fredrussell]

In one of his top tips in Yachting Monthly I recall Tom Cunliffe saying for a quicker working out, using rule of tenths is acceptable. I’ve done it that way for a while now and have yet to be caught out. Maths was never my strongpoint.

 

[Poignard]

In one of his top tips in Yachting Monthly I recall Tom Cunliffe saying for a quicker working out, using rule of tenths is acceptable. I’ve done it that way for a while now and have yet to be caught out. Maths was never my strongpoint.

The metric version. Damned unpatriotic, Sir!

 

[ithet]

No good on the river Hamble. Tide ambles in for about six hours, hangs around wasting time for two or three, suddenly realises it needs to be somewhere else and is all gone in about three hours.

 

[NormanS]

Yes

All the rule of twelths does is to produce an approximation of a sine wave based on high and low water heights and times. Bearing in mind that there are weather related variables anyway, it's usually good enough. But if you have a modern chart app such as navionics, you can get a minute by minute prediction from that with out the math.

Yes, but that's still just a prediction. There are lots of variables, like wind and atmospheric pressure. Most places where we sail the twelfths rule is perfect. It used to be even easier before metrification, when the tidal range was about 12 feet. ☺

 

[Neeves]

12 feet tides - unreal!!

LW395 - you have defined the issue - that I did not clearly describe.

I can cope with the maths, 12 times table and deviding by 12 is not too complex - having internet access is a different issue.

To me the rule of 12ths simply defines the height of water at, say, one hour after or 3 hours before low tide (or high tide). It assumes the tide time indicated is correct (and the time really does not matter (to the rule). You then calculate water height 1/12, for the first hour, 2/12th for the second etc. The calculation depends on the tide height being correct (and I know sustained strong winds or, air, pressure can impact the height -

but is 3/12ths considered a fair approximation for the end of the second hour - or has the sine wave been more accurately plotted.

To illustrate

The high tide yesterday was 1.93m, today 1.83m and tomorrow 1.7m.

So to ensure I could float of tomorrow - what allowance should I haver made - to dry out yesterday and float off tomorrow.

I confess I chickened out (hence the thread) I calculated for tomorrow - and came off today (without issue). If I had got it wrong and stayed, or better put stayed and tried to come off tomorrow - I'd have to wait a month if I cocked it up.

I appreciate that these tides don't figure highly for most in thew UK - but the inconvenience of getting it wrong is the same . Up north we do have 10m tides - different issues.

Jonathan

 

[prv]

but is 3/12ths considered a fair approximation for the end of the second hour - or has the sine wave been more accurately plotted.

It's presumably a fair approximation for a sine wave, but in plenty of places the tides aren't a sine wave to begin with. Approximating to the wrong thing isn't very helpful.

I use the Imray tides app - the interface is a little clunky but it has all the information that exists (or at least, is known to each country's hydrographic office) including the tidal curve for each standard and secondary location. You drag a little boat icon along the curve and read off the height of tide at that time. It's as accurate as you can get for a theoretical prediction that doesn't take weather etc into account.

Pete

 

[Refueler]

No good on the river Hamble. Tide ambles in for about six hours, hangs around wasting time for two or three, suddenly realises it needs to be somewhere else and is all gone in about three hours.

The Hamble is subject to the Southampton Water Double Tide .... so no-one would ever use 12ths there ... or 10ths ...

 

[JumbleDuck]

With the vagueness of tide charts modified by pressure and wind and the availability of tidal graphs - how dependable is the rule of 12ths?

 

[RichardS]

View attachment 83678

So "spot on" is the answer.

Richard

 

[Boathook]

View attachment 83678

To me that makes the rule of twelths accurate enough.

 

[noelex]

It's presumably a fair approximation for a sine wave, but in plenty of places the tides aren't a sine wave to begin with. Approximating to the wrong thing isn't very helpful.

I use the Imray tides app - the interface is a little clunky but it has all the information that exists (or at least, is known to each country's hydrographic office) including the tidal curve for each standard and secondary location. You drag a little boat icon along the curve and read off the height of tide at that time. It's as accurate as you can get for a theoretical prediction that doesn't take weather etc into account.
Pete

+1

Electronics are more accurate than simply applying the rule of twelfths.

Someone mentioned the River Hamble. A couple of clicks on my tablet gives the tidal prediction (it is important to remember this is a prediction and does not take into account all factors such as wind and pressure ).

It is very quick and simple.

The rule of twelfths is worth knowing and a manual backup is a good idea in the case of electronic falure, although this is increasing unlkely with the plethora of devices that can display this information.

 

[rogerthebodger]

The rule of 12th in its self if pretty accurate but interpolation calculation that can trip people up including me.

We are required in a skipper exams to use the rule of 12th but I fing using a tide chart even through our tides are mainly sinusoid as I find it easier and the same procedure if you get into an area with complex tides like the UK